Ammentos is a lightweight persistence framework for JDK 5. It does not require any installation nor configuration; just put a jar file into your classpath and start writing code. It is designed so that your persistence code will be dramatically short and so that you won't have to spend a lot of time to learn how to use it. It is about 72Kb large and it does not require any external library except for your database JDBC driver. You can use it in desktop applications or in server-based environments.
With MetaModel, you use a type-safe SQL-like API for querying any datastore. It is a data access framework providing a common interface for exploration and querying of different types of datastores. It isn't a data mapping framework. Instead, it emphasizes abstraction of metadata and the ability to add data sources at runtime, making MetaModel great for generic data processing applications, but less so for applications modeled around a particular domain.
BeanKeeper is a full featured, powerful object/relational persistence and query library. It has a unique philosophy of simpleness and ease of use: it does not require config files, annotations, code generation, code "enhancing", or any kind of tools to be used. It handles simple Java beans (or POJOs) and its query service returns lists. Despite its simpleness, under the hood the library takes care of all the necessary logic: transaction visibility, result list paging, keys and ids, table creation/modification, class relations, inheritance/polymorphism, case sensitiveness, and much more. It currently supports: Oracle, HSQLDB, MySQL, and PostgreSQL.
Druidedb is a simple XML file database API in Java to manage an XML database file system without database complexity (no server or client, and stores only strings). This API creates both the access DAO sources files and the DB filesystem. It can handle auto-increments and primary keys.
Groovy is an agile, dynamic language for the JVM which combines many features from languages like Python, Ruby, and Smalltalk and makes them available to Java developers using a Java-like syntax. It is designed to help get things done on the Java platform in a quicker, more concise, and fun way. It can be used as an alternative compiler to javac to generate standard Java bytecode to be used by any Java project or it can be used dynamically as an alternative language, such as for scripting Java objects, templating, or writing unit test cases.
HeliDB is a database for storing key-value pairs from a Java program. The client program creates one database object per database file. The database objects can be configured to support different capabilities. In its simplest configuration, a database is a file-backed map. More advanced configurations can give it transaction support or fast record searches by indexing records in a B+ tree.